1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic process applying development by use of a two-component developer.
2. Description of Related Art
As an image forming apparatus of an electrophotographic process, heretofore, known is an image forming apparatus mounted with a developing apparatus of a so-called trickle development process, in which a two-component developer comprising a toner and a carrier (hereinafter, also referred to as simply “a developer”) is utilized, a toner triboelectrically charged by stirring with a carrier being conveyed to a development region, a toner image being formed by making a toner adhere on an electrostatic latent image formed on an image carrier, and further a used developer is gradually removed as well as an unused toner and carrier are continuously or intermittently supplied.
Generally, a two-component developer receives a stress due to a stirring motion to make additives such as a lubricant having been added on the surface of toner particles constituting a toner release from or contrary be buried into toner particles, as a result, charging ability of a developer may be deteriorated to decrease the charge amount of a toner, or fluidity of a developer may be decreased to make coefficient of friction between a carrier and a toner high to cause a phenomenon of carrier contamination due to adhesion of a toner on a carrier; however, excessive deterioration of a developer is restrained because a unused toner is continuously or intermittently supplied as well as additives will be also supplied thereby by adopting such a trickle development process. Herein, it is difficult to make additives uniformly contact with a toner and a carrier even when additives are independently supplied as they are into a development apparatus.
However, in such a trickle development process, since a toner and a carrier are stored in the same vessel in advance and a carrier is supplied according to supply control of a toner, there is a problem that a supply amount of a carrier cannot be directly controlled. Therefore, a supply amount of a carrier varies corresponding to a printing ratio (hereinafter, referred to as “a coverage”) of an original. That is, since a toner amount supplied for image formation is large in the case of continuously printing an original having a high coverage, there is a problem that a supply amount of a carrier which is supplied simultaneously with toner supply becomes large to increase CCP (Cost Per Print: a cost per one sheet of printing paper). Further, in the case of continuously printing an original having a low coverage, since the amount of a toner supplied for image formation is very small, the supply amount of a carrier supplied simultaneously with toner supply also decreases to decrease a removal amount of a developer, which decreases an exchange amount of a carrier resulting in deterioration of a carrier, there is a problem of generation of image deterioration such as fogging and a toner scattering due to decrease of fluidity and charging ability of a developer.
To solve the above-described problems, technologies such as disclosed in JP-A 9-204105 (hereinafter, JP-A refers to Japanese Patent Publication Open to Public Inspection No.), JP-A 10-63074 and JP-A 2006-71859 are proposed.
In 2-A 9-204105 and JP-A 10-63074, as shown in FIG. 11, proposed is an image forming apparatus equipped with development apparatus 50 which supplies a toner stored in toner storing part 51 and a carrier stored in carrier storing part 52 to developer storing part 53 while independently controlling their supply amounts. In such an image forming apparatus, although a supply amount of a carrier can be directly controlled, sufficient triboelectric charging of a toner by stirring with a carrier cannot be obtained because a toner and a carrier are supplied into developer storing part 53 without being mixed or stirred in advance, which causes scatter of a charging amount between a toner supplied and a toner stored in development storing part 53 to make charging amount distribution of a toner broad, whereby there is a problem of generation of image deterioration such as fogging.
Further, in JP-A 2006-71859, proposed is an image forming apparatus equipped with development apparatus 60 in which a toner stored in toner storing part 61 and a carrier stored in carrier storing part 62 are temporarily stored in intermediate tank 64 while independently controlling the supply amounts, and a toner and a carrier are stirred while being circulated in this intermediate tank 64 to be supplied to developer storing part 63 as a supplying developer. However, in this image forming apparatus, it is difficult to rapidly supply a supplying developer of a varying trickle rate (a carrier concentration of a supplying developer) to developer storing part 63 unless the whole of a supplying developer stored in intermediate tank 64 is removed. Therefore, since supply control of a carrier cannot be minutely performed, supplying response of a carrier is insufficient and increase of CCP and deterioration of a carrier cannot be sufficiently dissolved. Further, there is also a problem that an apparatus inevitably becomes big due to installation of intermediate tank 64.
This invention has been made in view of the above-described problems, and the object is to provide an image forming apparatus which exhibits sharp charging amount distribution of a toner, restrains deterioration of a developer because of excellent supplying response of an adequate amount of a toner and a carrier, and enables to stably form high quality images for a long term.